这个程序要在一个array里面添加数字， 删除数字， 算array里的以用空间，但是我现在就测试了一下加数字，然后我要输出这个数字，为什么我输入完数字以后程序就结束了，是我的toString有问题，还是add这个method有问题？ 高手帮我看一下

public class IAS
{
   private int [] a=new int[2];
   
    /*public IAS ()
    {
    }*/
   
    public void add(int num)
    {
        int i=0, size, count=0;
        do
        {
            if(a[i]==0)
            {
               a[i]=num;
               count++;
            }
            else if(a[i]!=0 && i<a.length)
               i++;
            else if(i>=a.length)
            {
               size=a.length*2;
                int a[]=new int[size];
            }
        }while (count==0);   
    }
   
    public void add(int num1, int num2, int num3, int num4)
    {
          int i=0, size, count=0;
        do
        {
            if(a[i]==0 && i+4<a.length)
            {
               a[i]=num1;
                a[i+1]=num2;
                a[i+2]=num3;
                a[i+3]=num4;
               count++;
            }
            else if(a[i]!=0 && i<a.length)
               i++;
            else if(i+4>=a.length)
            {
               size=a.length*2;
                int a[]=new int[size];
            }
        }while (count==0);
    }
   
    public void clear()
    {
       int a[]=new int[2];
        for (int i=0; i<2; i++)
            a[i]=0;
    }
   
    public boolean contains(int num)
    {
        int count=0;
        
        for(int i=0; i<a.length; i++)
        {
            if(a[i]==num)
               count++;
        }
        
        if(count>0)
            return true;
        else
            return false;
    }
   
    /*public IAS copy()
    {
       int size=a.length;
        int[] array=new array[size];
        for(int i=0; i<size; i++)
            array[i]=a[i];
        return array[];        
    }*/
    /*
    public boolean equals(IAS array)
    {
        int count=0;
        
        for(int i=0; i<a.length; i++)
        {
            if(a[i]==array[i])
               count=0;
            else
               count++;
        }
        if(count==0)
           return true;
        else
           return false;
    }*/
   
    public boolean isEmpty()
    {
       int count=0;
        for(int i=0; i<a.length; i++)
        {
            if(a[i]!=0)
               count++;
        }
        if(count==0)
           return true;
        else
           return false;
    }
   
    public void remove(int num)
    {
        for(int i=0; i<a.length; i++)
        {
            if(a[i]==num)
            {
               for(int j=i; j>=0; j--)
                   a[j]=a[j+1];
            }
        }
    }
   
    public int size()
    {
       int count=0;
        for(int i=0; i<a.length; i++)
        {
           if(a[i]!=0)
               count++;
        }
        return count;
    }
   
    public String toString()
    {
       String str= "{ " + a + " }";
        return str;
    }
}

        
import java.util.Scanner;

public class IAStest
{
   public static void main (String[] args)
    {
       int num;
       IAS one=new IAS();
        IAS two=new IAS();
        
        Scanner keyboard=new Scanner(System.in);
        
        System.out.print("Enter a number: ");
        num=keyboard.nextInt();
        
        one.add(num);
        one.toString();                                                  //当我输入完数字后程序就结束了，怎么回事？
    }
}